The present invention relates to an apparatus for forming microscopic recesses on a circumferential surface that defines a cylindrical bore in a workpiece and comes into sliding contact with a counterpart, and a method of forming microscopic recesses on the circumferential surface thereof by using the apparatus. More specifically, in order to reduce friction occurring on the circumferential surface, the present invention relates to an apparatus for forming microscopic recesses as oil retention portions on the circumferential surface, for instance, a cylinder bore surface of a cylinder block of an engine for automobiles, a cylinder bore surface of a compressor, a sliding surface of a cylindrical bore of a slide member, a bearing surface of a cylindrical bore of a sliding bearing and the like, and relates to a method of forming microscopic recesses on the circumferential surface by using the apparatus.
Conventionally, upon forming microscopic recesses on a circumferential surface that defines a cylindrical bore in a workpiece, the circumferential surface is subjected to shot blasting. Upon shot blasting, a masking sheet with through-holes having a predetermined shape is attached to the circumferential surface, and then small-diameter shots, such as ceramic balls, are blasted with compressed air against the circumferential surface. As a result, microscopic recesses are formed on portions of the circumferential surface which are exposed outside through the through-holes. Subsequently, the masking sheet is taken off, and the circumferential surface is subjected to cleaning and honing to thereby remove protrudent peripheral portions around the microscopic recesses which are formed upon shot blasting. Japanese Patent Application First Publication No. 2002-307310 describes such a masking and blasting method as explained above.
Further, Japanese Patent Application First Publication No. 2005-319476 corresponding to U.S. Patent Application Publication No. 2005/0245178 A1 describes a microscopic recesses forming apparatus which includes a rotatable tool holder and a microrecess-forming unit moveable in a direction perpendicular to a rotation axis of the tool holder. The rotation axis of the tool holder is located offset from the center of gravity of the microrecess-forming unit. Therefore, when the tool holder is rotated at high speed, centrifugal force exerted to the microrecess-forming unit excessively increases in proportion to the square of the rotation speed of the tool holder. This results in that a form roller of the microrecess-forming unit is pressed against a cylindrical bore surface of a workpiece at a high load. To overcome thus problem, the form roller must be rotated at a low speed in order to press the form roller against the cylindrical bore surface at the low load. This causes deterioration in working efficiency of the apparatus and thereby increase in production cost of the apparatus.